System and method for aligning internal transmit and receive clocks
A system includes a master device connected to one or more slave devices via a channel, the channel communicating an externally generated first system clock towards the master device. A delay locked loop circuit receives the first system clock and a second phase feedback signal as inputs and generates a transmit clock signal. A phase offset circuit receives the transmit system clock and generates a phase shifted version of the transmit clock signal as a second system clock. A first phase detector receives a receive system clock and the transmit system clock and generates a first phase feedback signal. A delay element receives the first system clock and the first phase feedback signal and generates a delayed first system clock. A second phase detector receives the delayed first system clock and the second system clock and generates the second phase feedback signal.
This application is a continuation of Ser. No. 09/499,025, filed Feb. 7, 2000, which is hereby incorporated by reference.BACKGROUND OF THE INVENTION
The present invention relates to a system and method for aligning two or more clock domains. More particularly, the present invention relates to a system and method for aligning transmit and receive clocks in a bus system.
In the example, shown in
The relationship between application 1 and master 3 is further illustrated in
The receive clock (rclk) in the master is normally aligned with the knowledge that data being sent from the slave devices is communicated in a known relationship to CTM, and that this relationship is maintained as both the data signals and CTM traverse the channel towards the master. In other words, the receive clock (rclk) is normally phase aligned in a known relationship to CTM. This relationship is designed to maximize the timing margin for sampling the data at master 3. In many contemporary bus systems, data is transmitted 90° ahead of its corresponding CTM edge. As illustrated in
To achieve the foregoing, DLL 4 may be used.
Referring to again to
In contemporary bus systems, it is common for data to be communicated 90° ahead of the corresponding CFM edge. Since there is a known, finite delay for the data traversing the output drivers in the master (output driver delay, TOD), achieving the desired data to tclk timing relationship requires that the transmit clock (tclk) be (90°+TOD) ahead of the corresponding CFM edge. This relationship is illustrated in
A clock recovery circuit yielding the desired tclk relationships is shown in
In addition to rclk and tclk, master 3 typically generates a third reference signal, Synclk. Synclk is used to control data exchanges between application 1 and master 3. That is, Synclk provides a reference for data signals received from the application by the master and for data signal sent from the master to the application. As illustrated in
Unfortunately, as suggested above, a great number of control and data signals in the master must necessarily be referenced to tclk instead of Synclk/rclk. The existence of separate tclk and rclk domains within a bus system creates a number of synchronization concerns. For example, data from the application to be transmitted by the master to one or more slave devices must first be received in the master. This application-to-master data transfer is done in accordance with Synclk. However, the data is transmitted from the master to the one or more slave devices in accordance with tclk. The transition of such data from the rclk domain to the tclk domain is accomplished by “holding” the data in the master for some defined period of time.
Following conventional theory, CFM and CTM are identical except for their propagation direction. Thus, rclk and tclk would be similarly related, but for the finite timing delays necessarily introduced by operation of the receiver and the output driver circuits.
Unfortunately, as described in greater detail below, the ideal relationship between rclk and tclk do not hold in practice. Rather, timing delays introduced by circuit operations in varying voltage and temperature condition tend to skew the phase relationship between rclk and tclk. Recognizing that the electrical circuits in issue here will vary in their response time across a range of process, operating, and environment conditions, bus system designers must necessarily expand the synchronizing “hold” time periods within the master for data to accurately transition between the rclk and the tclk domains.
The timing diagram of
Ideal sampling points for data transmitted from the application to the master correspond to the rising edge of rclk, as indicated by letters a, b, c, and d in
However, as practically implemented within contemporary bus systems, the actual sampling of this data occurs at the falling edges of tclk, as indicated by aa, bb, cc, and dd of
To summarize, the setup time requirement for the data can be described as:
the hold time requirement for the data can be described as:
In actual implementation, however, the output driver delay (TOD) is seldom equal to (90°−TSETUP
A method of aligning clock signals in a bus system includes generating a transmit clock signal in a master, and arbitrarily adjusting the phase of the transmit clock signal while maintaining a first predefined phase relationship between the transmit clock signal and a second system clock. A further adjustment of the phase of the transmit clock signal may be made to have a second predefined phase relationship with a receive clock signal while maintaining the a first predefined phase relationship between the transmit clock signal and the second system clock. In one embodiment, the second predefined phase relationship between the transmit clock signal and the receive clock signal is 180°.
In another aspect, a method of aligning clock signals in a bus system includes generating a transmit clock signal in a master in relation to a first system clock, shifting the transmit clock signal phase by 90°, and passing the phase shifted transmit clock signal through an output driver circuit in the master to generate a second system clock. As a result and in contrast to the conventional expectation, the first and second system clocks need not be phase aligned.
In yet another aspect, the present invention provides a method of aligning system clocks in a bus system by generating a first system clock external to the master such that the first system clock propagates via the channel through the one or more slave towards the master, and generating in the master a second system clock having a phase relation to the first system clock defined such that, the phase difference between the first system clock and the second system clock is substantially equal to 90° minus the sum of the receiver setup delay and the output driver delay.BRIEF DESCRIPTION OF THE DRAWINGS
The maximum effective operating speed for a bus system is essentially the sum of critical path timing requirements. Further, data robustness in the bus system is a product of timing margins. Timing margins are impacted by a host of timing requirements. The restrictive setup and hold requirements explained above disadvantageously impact effective operating speed and timing margins.
The present invention addresses this problem by providing a system and method in which an ideal phase relationship between tclk and rclk domains can be maintained for all output driver delays across a range of bus system operating conditions. In one aspect, the present invention utilizes a CFM driver circuit which allows for arbitrary phase adjustments of tclk while maintaining the correct phase relationship between tclk and CFM, i.e., tclk being (90°+TOD) ahead of CFM. Thereafter, the phase of tclk may be further adjusted until it has an optimal phase relationship with rclk, i.e. tclk being separated from rclk by 180°.
The circuit shown in
The output of DLL 20 also passes through buffer 22b to yield tclk which is applied to the data output drivers 24a, 24b, . . . 24n corresponding to Data 0, Data 1 . . . Data n. Along with rclk, the complement of tclk is applied to ZPD 26.
The circuit shown in
An alternative circuit is shown in
The exemplary circuits shown above may be modified to operate by using the complement of rclk, rather than tclk to control the output drivers. Since the feedback loop in the circuits above aligns tclk to the complement rclk, either signal may be used to control the transmit circuitry. Where the complement of rclk is used as the controlling signal, tclk exists merely to produce CFM.
All of these techniques yield the clock relationships shown in
where TOD equals the output driver delay and TSETUP
With these desired relationships established, the application of the related clock signals to the devices in the bus system will now be examined. As can be understood from reference to system configuration illustrated in
Total Delay=Intrinsic Delay+Cycle Delay+Fractional Delay.
Intrinsic delay is the time required to decode and execute an instruction at a slave device and does not vary between slave devices. For example, where the bus system is a memory system, intrinsic delay is the time required to decode an incoming “Read” request packet and fetch the desired data from memory.
Fractional delay is the extra delay that a slave device adds to the intrinsic delay such that the output of the desired data will be correctly aligned to the transmit clock (CTM). This delay linearly varies from zero when a slave device is near the upper end of a CTM/CFM cycle boundary to one cycle when a slave device is near the lower end of a CTM/CFM cycle boundary. As the CTM/CFM skew passes through a cycle boundary, the fractional delay value is reset to zero.
In the example illustrated in
A detailed circuit capable of introducing the fractional delay noted above has previously been described in commonly assigned U.S. Pat. No. 6,473,439, the subject matter of which is incorporated herein by reference. Whatever circuit actually used to achieve the desired results above, the concept of cross clock domain transition (i.e., fractional delay adjustment between receive and transmit clock domains) is illustrated in
Since CTM and CFM can have any phase relationship, care must be taken when passing data from the received clock domain (indicated by the dotted line in
In one preferred embodiment, the clock domain transition circuit 34 chooses between two different delay paths based on the relative phases of CTM and CFM, such that setup and hold requirements in the transmits data block 36 are not violated. The transitions between these two delay paths occur at the CTM/CFM phase intervals of n*tcycle and (n+0.5)*tcycle. The first of these transitions causes the fractional delay to reset from one to zero. The second transition is required for correct circuit operation, but is not externally visible.
In conventional bus systems, the phase difference between CTM and CFM at a given slave device did not change appreciably. Rather, it was fixed by the length of the trace between the master and the slave device, as well as the propagation delay through the master. Accordingly, conventional bus systems would only activate the “Self Transition” function once during system initialization. During Self Transition the correct fractional delay would be determined, and based on an observation of received data at the master, for example, the cycle delay register would be programmed, such that each slave device presented the same apparent delay.
In contrast, the CTM and CFM phase difference resulting from application of the concepts of the present invention will vary according to operating conditions, i.e., changes in TOD as a result of temperature, voltage etc. Thus, slave devices must be able to compensate for the changing phase relationship. There are a number of techniques which competently address this new requirement.
In a first technique, each slave device recalculates its fractional delay with sufficient frequency to effectively compensate for any variation in TOD. This technique works well for bus systems whose total round trip is less than one cycle, because the update will require little controller overhead. However, systems exhibiting delays greater than one cycle are problematic because the apparent delay for slave devices near n*tcycle boundaries may change as the CFM to CTM phase relationship shifts. To compensate for this effect, the master would necessarily measure the delay for data arriving from each slave device following fractional delay adjustment, and reprogram, as necessary, the cycle delay register to maintain a constant apparent delay. Unfortunately, the overhead required to dynamically adjust both fractional and cycle delay components in this manner is prohibitive for many bus system applications.
Thus, in a preferred approach to this cycle boundary crossing problem, the slave device detects when it crosses a cycle delay boundary, and increments or decrements the cycle delay value in the cycle delay register accordingly. Such detection may be accomplished by noting when the fractional delay value goes back and forth across the 0 and 1 boundary.
In a second technique, sufficient margin is provided in the slave device CTM/CFM phase calibration circuitry to handle the TOD variation. Contemporary fractional delay circuits can automatically track up to 0.1*tcycles of CFM to CTM variation following operation of the Set Transition function. Further, variations in TOD may be significantly reduced by isolating the master (or master interface circuit) from environmental factors such as temperature and voltage.
A third technique is illustrated in
More specifically, CTM is applied to DLL 40 and delay line 46. The output of DLL 40 is applied to 90° block 41 and output through buffer 42b as tclk. The output of 90° block 41 passes through buffer 42a as tclk 90° and an output driver circuit 43 as CFM. A first zero phase detector circuit 45 receives rclk and the complement of tclk as inputs and also drives delay line 46. The output of delay line 46 and CFM are input to a second ZPD 47 which drives DLL 40.
1. A method of aligning clock signals in a system comprising a master and one or more slave devices connected via a channel, the system further comprising a first system clock propagating towards the master and a second system clock propagating away from the master, and the master further comprising a transmitter and a receiver, the method comprising:
- generating a transmit clock signal in the master, wherein data is driven onto the channel in relation to the transmit clock signal; and
- adjusting the phase of the transmit clock signal in the master while maintaining a first predetermined phase relationship between the transmit clock signal and the second system clock.
2. A method of aligning clock signals in a system comprising a master and one or more slave devices connected via a channel, the system further comprising a first system clock propagating towards the master and a second system clock propagating away from the master, the method comprising:
- generating a transmit clock signal in the master in relation to the first system clock;
- shifting a phase of the transmit clock signal phase by a predefined offset; and
- passing the phase shifted transmit clock signal through an output driver circuit in the master to generate the second system clock, such that the first and second system clocks are not phase aligned.